1. Technical Field
The present invention relates to an access control method and an access control arrangement to control access to a communication channel with a plurality of units coupled to it.
2. Discussion of Related Art
Such a method and arrangement are already known in the art, e.g. from the European Patent Application, publication number 0562 222 A1, "Access control arrangement". Therein, the access of a plurality of interconnected units to a communication channel is controlled by a mechanism whereby each of the interconnected units receives from a preceding unit an access request word comprising a plurality of successive request channels to which priorities are assigned, and whereby each unit writes its own priority in the corresponding request channel. The request word is then transmitted to a following unit. Before writing its own priority in the request channel, each unit compares its own priority with the priorities of the preceding units, this comparison resulting in an withdrawal of this unit from the communication channel in case its own priority is lower than the priorities of preceding units, and resulting in an inhibition of access of these preceding units having a lower priority than the unit under consideration. When after a predetermined time interval the unit under consideration is itself not inhibited by any of the following units, access of this unit to the communication channel is granted. This mechanism is thus not only based on a priority, but also on the physical position of the unit with respect to a control unit which has a reference position, thereby determining the meaning of "preceding" or "following". Indeed, in case several units have identical priorities, the unit which is located at the largest distance from the control unit, gains the access. This means that after a first priority phase whereby each unit compares priorities and possibly puts its own priority in the request channel, a positional phase grants the access to the communication channel as a function of the physical position of the unit with respect to the controller. In the described prior art arrangement this latter mechanism is realized by a control line carrying the access flag signal signal UAF, which interconnects the units in a daisy-chain connection. This access flag signal is determined individually per unit, based on the access flag signal generated in the preceding unit. The daisy-chain configuration however implies that, in case a unit is removed from the communication channel, extra control logic circuitry is to be provided to maintain the functioning of this control line for generating the appropriate access flag signal UAF per unit, based on the signal generated in a preceding unit, For an embodiment of the arrangement whereby all individual units are coupled to a backpanel, this proper functioning of the access mechanism therefore requires extra logic circuitry to be added to the backpanel. This however enhances the failure rate of the complete arrangement, whereby the latter configuration also seriously enlarges the mean time to repair the arrangement in case of malfunctioning of this logic circuitry, this fact representing a major drawback.
A second major drawback of the prior art arrangement is the limit in units that can be interconnected to the communication channel. Indeed with the current signal and clock levels at the time of the invention, a maximum of 32 interconnected units is a physical limit for the prior art configuration.